a high-efficiency automatic de-bubbling system for …mit.imt.si/izvodi/mit166/kuo.pdf · 2016. 12....

C.-C. KUO, C.-M. HUANG: A HIGH-EFFICIENCY AUTOMATIC DE-BUBBLING SYSTEM FOR LIQUID SILICONE RUBBER995–1000

A HIGH-EFFICIENCY AUTOMATIC DE-BUBBLING SYSTEM FORLIQUID SILICONE RUBBER

VISOKOZMOGLJIV SISTEM ZA ODPRAVLJANJE MEHUR^KOV VTEKOÎ SILIKONSKI GUMI

Chil-Chyuan Kuo, Chuan-Ming HuangMing Chi University of Technology, Department of Mechanical Engineering, No. 84, Gungjuan Road, Taishan Dist. New,

Taipei City 24301, [email protected]

Prejem rokopisa – received: 2014-06-13; sprejem za objavo – accepted for publication: 2016-10-27

doi:10.17222/mit.2014.089

The silicone-rubber mold is regarded as an important method for reducing the cost and time to market a new product byshortening the development phase. A commercial, automatic, vacuum machine is widely used to degas in the manufacturing of asilicone-rubber mold, but the hardware is costly. A low-cost, high-efficiency degassing system was designed and implementedfrom a regular vacuum machine. The control style was based on a human-machine interface. It was found that the wholedegassing-process sequences consist of the explosive phase, the balanced phase and the convergence phase. The time saving inthe degassing process can be at least 42 %. Six predicted equations for both the balanced phase and the convergence phase areinvestigated and the maximum relative error of these equations can be controlled to within 6.34 %. The advantages of thedeveloped de-bubbling system include saving labor, reducing the human error of the operator and a higher degassing efficiency.Keywords: air bubbles, de-bubbling, silicone rubber mold, rapid tooling

Forma iz silikonske gume je pomembna pri zmanj{evanju stro{kov in skraj{anju ~asa razvojne faze pri uvajanja novega izdelkana trg. Pri izdelavi forme iz silikonske gume se za razplinjevanje uporabljajo drage komercialne vakuumske avtomatske naprave.Iz obi~ajne vakuumske naprave je bil izdelan poceni in u~inkovit razplinjevalni sistem. Kontrola stroja temelji na povezavi~lovek-naprava. Ugotovljeno je, da razplinjevanje sestoji iz eksplozivne faze, faze uravnoteènja in iz faze konvergence,Postopek razplinjevanja omogo~a 42 % prihranek ~asa. Preiskanih je bilo {est predvidenih ena~b pri obeh uravnoteènih fazahin pri konvergen~ni fazi in maksimalna napaka zna{a 6,34 %. Prednosti razvitega sistema razplinjevanja so: prihranek dela,zmanj{anje ~love{kih napak operaterja in ve~ja u~inkovitost razplinjevanja.Klju~ne besede: zra~ni mehur~ki, odprava mehur~kov, forma iz silikonske gume, hitra izdelava orodij

1 INTRODUCTION

To reduce the time and the cost of product develop-ment, rapid prototyping (RP) was developed.1 This offersthe potential to completely revolutionize the process ofmanufacture. However, the features of the prototype donot usually meet the needs of the end product with therequired material. Rapid tooling (RT) technologies arethen developed because it is the technology that uses RPtechnologies and applies them to the manufacturing ofmold inserts.2 Since the importance of RT goes farbeyond component performance testing, RT is regardedas an important method of reducing the costs and thetime to market in a new-product development process.Several RT technologies are commonly available inindustry now. RT is divided into direct tooling and indi-rect tooling.3 Direct tooling means fabricating moldinserts directly from an RP machine, such as selectivelaser sintering.4,5 Indirect tooling means fabricating themold insert by a master pattern fabricated using variousRP technologies. Soft tooling is used for low-volumeproduction. The materials used for soft tooling have lowhardness levels, such as silicone-rubber6 and epoxy-resincomposites.7 Conversely, hard tooling is associated with

higher volumes of production. Materials used for hardtooling often have high hardness levels. Soft tooling iseasier to work with than tooling steels because thesetools are created from materials such as epoxy-basedcomposites with aluminum particles, silicone rubber orlow-melting-point alloys. It is well known that RT iscapable of replacing conventional steel tooling, savingcosts and time in the manufacturing process.8 Indirectsoft tooling is used more frequently in the developmentof new products than direct tooling, because it is fast,simple and cost-effective. It is a well-known fact that asilicone rubber mold is employed frequently because ithas flexible and elastic characteristics, so that parts withsophisticated geometries can be fabricated.9 A silicone-rubber mold can be used for producing low-melting-point metal parts, wax patterns and plastic parts. Airbubbles in the silicone-rubber mold are one of the mostcommon types of defects, especially in the vicinity of themaster pattern. A silicone-rubber mold with air bubblesappearing in the vicinity of the master pattern willchange the appearance and dimensional accuracy of thepart duplicated from this silicone-rubber mold usingvacuum casting. Conventionally, de-bubbling the airbubbles with a purely manual operation mode depends

Materiali in tehnologije / Materials and technology 50 (2016) 6, 995–1000 995

UDK 677.017.7:678.84:678.032 ISSN 1580-2949Professional article/Strokovni ~lanek MTAEC9, 50(6)995(2016)

significantly on the experiences of the operator. Thedrawbacks of this method include human error and noisepollution derived from the vacuum pump. A com-mercially available automatic vacuum machine waswidely employed to degas in the manufacturing of sili-cone-rubber molds, but the hardware is costly.10 Inaddition, the programmable logic controller mode lacksthe flexibility to modify the program. Hence, developinga low-cost and easy-to-operate automated de-bubblingsystem is a major concern. To meet this requirement, theobjective of this work is to develop a low-cost andhigh-efficiency automatic de-bubbling system with ahuman-machine interface (HMI).11 Three vessels wereused for filling the liquid silicone rubber for de-bubbling.The de-bubbling process sequences were investigated indetail. The effect of the pressure-relief process in theexplosive phase on the de-bubbling efficiency for manualand automatic modes was also analyzed. Trend equationsfor predicting the balance phase duration and conver-gence phase duration for three vessels were investigated.The performance of the automatic de-bubbling systemdeveloped was evaluated. Comparisons of the de-bubbl-ing efficiencies for automatic and manual modes werecompared.

2 EXPERIMENTAL PART

Figure 1 shows a low-cost, automated de-bubblingsystem with an HMI. This system consists of a photo-electric sensor (EX-11EB; SUNX, Inc.), a programmablelogic controller (PLC) (FX 2N-32MR, Mitsubishi), anHMI (GP37W2-BG41-24V; Pro-face, Inc.), an electro-magnetic valve (SUG 15-24VDC; Chelic, Inc.) and an

electronic buzzer (TS2BCL; Tend, Inc.). A photoelectricsensor (response time 0.5 ms) was used for detecting theair bubbles during the de-bubbling process. A PLC wasused as a key component of the electric control module.An HMI was used for operating the automatic de-bubbling system. The HMI consists of all the aspects ofthe interaction and communication between the operatorsand machines by using a graphical HMI unit. Theelectromagnetic valve (on-off reaction time < 15 ms) wasused to break the vacuum automatically. An electronicbuzzer was used to alert the operator when the de-bubbling process is completed. Considering the practicalrequirements for the different operators, the de-bubblingmethod of this system includes an automation mode anda manual control mode. This system can be equippedwith manual and automatic modes. Three different volu-mes (250 mL, 500 mL and 1000 mL) of vessels wereused for filling the liquid silicone rubber in this study.The silicone rubber (KE-1310ST; ShinEtsu, Inc.) in theliquid state was mixed with a hardener (CAT-1310S;ShinEtsu, Inc). Generally, the curing agent and silicone

C.-C. KUO, C.-M. HUANG: A HIGH-EFFICIENCY AUTOMATIC DE-BUBBLING SYSTEM FOR LIQUID SILICONE RUBBER

996 Materiali in tehnologije / Materials and technology 50 (2016) 6, 995–1000

Figure 2: Brief flowchart of the automatic vacuum degassing processusing an HMISlika 2: Shema poteka postopka avtomatskega vakuumskega razpli-njevanja s pomo~jo HMI

Figure 1: A low-cost, automated, de-bubbling system with an HMISlika 1: Stro{kovno ugoden in avtomatiziran sistem za odpravo me-hur~kov s HMI

rubber in a weight ratio of 10:1 were mixed thoroughlywith a stirrer. After the de-bubbling process, the pressureinside the vacuum machine was changed by breaking thevacuum atmosphere. Thus, a silicone-rubber mold can befabricated with defects caused by the air bubbles derivedfrom the mixing process. To reduce the difference causedby the different operators in the amount of air bubbles,while mixing the liquid silicone rubber, an agitationblade for mixing the liquid silicone rubber was designedand fabricated. To confirm the center of the vessel isaligned with the center of the agitation blade, apositioning fixture was designed and fabricated. Figure2 shows a brief flowchart of the automatic vacuumdegassing process using an HMI. Due to theexperimental limitations, the solenoid valve does notwork when the break vacuum duration is set less than0.03 s. Thus, the break vacuum duration was set to be0.03 s.

3 RESULTS AND DISCUSSION

The center of the vessel can be aligned with thecenter of the agitation blade using the positioning fix-ture. Figure 3 shows the three phases of the de-bubblingprocess sequences. In general, the liquid silicone rubberhas a large number of air bubbles because of the mixingreaction of the materials. As can be seen from this figure,

the first phase is the explosive phase, where the volumeof liquid silicone rubber is drastically increased becausethe pressure in the vacuum chamber is lower than theatmospheric pressure, as shown in Figure 4a. Thesecond phase is the balance phase, where the air bubblesin the liquid silicone rubber are eliminated gradually, asshown in Figure 4b. The final phase is the convergencephase, where the number and the size of the air bubblesin the liquid silicone rubber are eliminated significantly,as shown in Figure 4c. Schematic illustrations of thethree phases of the de-bubbling process are shown inFigure 5. A number of air bubbles less than 15 is definedas the end of balance phase. In addition, no more airbubbles inside the liquid silicone rubber is defined as theend of convergence. A stopping vacuum of 120 s bet-ween convergence phase and convergence phase isrequired for reducing the convergence phase’s duration.This is because air bubbles inside the liquid siliconerubber can reach the top of the liquid silicone rubberduring 120 s. The air bubbles can be eliminated comple-tely by an automatic de-bubbling system with an HMIbased on the criteria discussed above.

Table 1: Time saving of the explosive phase for different volumes ofsilicone rubber

Table 1: Prihranek ~asa pri eksplozivni fazi, pri razli~nih prostorninahsilikonske gume

Volume ofsilicone

rubber (mL)

Percentageof volume

(%)Mode

Explosivephase du-ration (s)

Timesaving

(%)

350 70Manual 5335

61.46Automatic 2056

300 60Manual 3952

81.12Automatic 746

250 50Manual 2883

82.59Automatic 502

200 40Manual 706

70.00Automatic 233

150 30Manual 283

61.48Automatic 109

100 20Manual 0

0Automatic 0

Table 1 shows the time saving of the explosive phasefor different volumes of silicone rubber. As can be seen,



Figure 4: Schematic illustrations of the three phases of the de-bubbl-ing process: a) explosive phase, b) balance phase and c) convergencephaseSlika 4: Shematski prikaz treh faz postopka razplinjevanja: a) eksplo-zivna faza, b) ravnote`na faza in c) konvergen~na faza

Figure 3: Three phases of the de-bubbling process: a) explosive phase, b) balance phase and c) convergence phaseSlika 3: Tri faze postopka razplinjevanja: a) eksplozivna faza, b) ravnote`na faza in c) konvergen~na faza

the maximum time saving of the explosive phase is about81.12 %. The time savings of the explosive phase in-crease with the increasing percentage of volume, rangingfrom 30 % to 60 %. The time saving of the explosivephase decreases when the percentage of volume exceeds60 %. These results show that the vessel with a percent-age of volume of 60% of silicone rubber is the optimumvalue.

Figure 5 shows the sequential procedures of theautomatic de-bubbling process. Figure 6 shows thesequential procedures of the de-bubbling process withmanual operation. It is obvious that the results for thetwo operation modes are the same, showing no airbubbles inside the liquid silicone rubber. This resultshows that the air bubbles inside the liquid siliconerubber can be eliminated completely with the automaticoperation mode. This means the system can be used forthe production of a high-quality, bubble-free, silicone-rubber mold.12

Precise determination the balance phase’s durationand the convergence duration is an absolute requirementfor a high-efficiency, automatic, de-bubbling process.Figure 7 shows the trend equations for the balanceduration and convergence duration for vessel volumes of250 mL, 500 mL and 1000 mL. For a vessel volume of250 mL, the balance phase’s duration (y) can be pre-dicted from the trend equation of y = 3.316x – 30.8 by

the volume of silicone rubber (x). Note that the R2 repre-sents the correlation coefficient. Generally, a higher R2

value (maximum value =1) means a better accuracy ofthe trend equation.16 Six predicted equations for both thebalanced phase and the convergence phase are investi-gated, and the maximum relative error of these equationscan be controlled within 6.34 %. This means that boththe balanced phase duration and the convergence phaseduration can be calculated from these equations.

To evaluate the performance of the automatic degass-ing system developed, each test was carried out threetimes with the mean and the deviation determined. Table2 shows the time saving of the total degassing time forthree different volumes of silicone rubber. Figure 8shows the total degassing time as a function of siliconerubber for the manual and automatic operation modes.As can be seen, a reduction in the degassing time of atleast 42 % can be observed using the automatic degass-ing system with a human-machine interface. It is obviousthat there is an increase in time saving of degassing withan increase in the volume of silicone rubber. Threephases are important for the degassing process, but theexplosive phase is the most critical one. This is becausethe pressure-relief process in the automatic operationmode is significantly different from that in the manualoperation mode, as shown in Figure 9. The recovery



Figure 6: Sequential procedures of the de-bubbling process with themanual de-bubbling mode: a) liquid silicone rubber before de-bubbl-ing process, b) explosive phase, c) balance phase, d) pause 120 s,e) convergence phase and f) de-bubbling process was completedSlika 6: Sekven~ni postopki postopka razplinjevanja pri ro~nem vo-denju razplinjevanja: a) teko~a silikonska guma pred razplinjevanjem,b) eksplozivna faza, c) ravnote`na faza, d) pavza 120 s, e) konver-gen~na faza in f) zaklju~en postopek razplinjevanja

Figure 5: Sequential procedures of the automatic operation mode:a) liquid silicone rubber before de-bubbling process, b) explosivephase, c) balance phase, d) pause 120 s, e) convergence phase andf) de-bubbling process was completedSlika 5: Sekven~ni postopki pri avtomatskem na~inu dela: a) teko~asilikonska guma pred razplinjevanjem, b) eksplozivna faza, c) ravno-te`na faza, d) pavza 120 s, e) konvergen~na faza in f) zaklju~enpostopek razplinjevanja

time needed for the pressure of the chamber reaching thedegassing pressure with the automatic operation mode is

shorter than that with the manual operation mode. Basedon the experimental results, the advantages of this sys-tem include saving labor, reducing the human error ofthe operator and a higher degassing efficiency. Thissystem has broad application prospects in the develop-ment of new products using a silicone-rubber mold.

4 CONCLUSIONS

A low-cost, high-efficiency degassing system hasbeen designed, implemented and tested in this study. Theentire degassing-process sequences consist of theexplosive phase, the balance phase and the convergencephase. The automatic degassing method provides threedecisive advantages compared with the manual degass-ing method. The explosive phase has been proved to be akey process for a high-efficiency degassing process. Areduction of the degassing time by at least 42 % can begained. This system has broad application prospects inthe development stage for new products using rapid-tool-ing technology.

Acknowledgement

This work was financially supported by the Ministryof Science and Technology of Taiwan under contractnos. NSC 102-2221-E-131-012 and NSC 101-2221-E-131-007.



Table 2: Time saving of the total degassing time for three different volumes of silicone rubberTabela 2: Prihranek ~asa od vsega ~asa razplinjevanja, pri treh razli~nih prostorninah silikonske gume

Volume of siliconerubber (mL) Mode

Explosivephase duration

(s)

Balance phaseduration (s) Pause (s) Convergence

phase duration (s) Total (s)Time saving

(%)

110Manual 1642 338 120 237 2337

45.64Automatic 583 334 120 233 1270

240Manual 1415 509 120 427 2471

42.24Automatic 416 496 120 396 1428

430Manual 1409 530 120 465 2525

42.07Automatic 406 508 120 428 1462

Figure 8: Total degassing time as a function of silicone-rubber vol-ume for manual and automatic operation modesSlika 8: Celotni ~as razplinjevanja (v odvisnosti od volumna) silikon-ske gume pri avtomatskem in ro~nem na~inu upravljanja

Figure 9: Schematic illustrations of the pressure relief process in theexplosive phase: a) manual operation mode and b) automatic operationmodeSlika 9: Shematski prikaz postopka spro{~anja tlaka v eksplozivnifazi: a) ro~ni na~in vodenja in b) avtomatski na~in vodenja

Figure 7: Trend equations of: a) balance phase duration and b) con-vergence phase duration for vessel volumes of 250 mL, 500 mL and1000 mLSlika 7: Trend ena~b za: a) trajanje ravnote`ne faze in b) trajanje kon-vergen~ne faze pri prostornini posode 250 mL, 500 mL in 1000 mL

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